1. Field of the Invention
The present invention relates to a vehicle front/rear wheel steering angle control apparatus and a vehicle front/rear wheel steering angle control method.
2. Description of the Related Art
A four-wheel vehicle is normally steered by adopting the front wheel steering method in which the front wheels alone can be steered. However, by adopting the front/rear wheel (four wheel) steering method in which the rear wheels can be steered as well, the minimum turning radius can be reduced and the cornering attitude while the vehicle is traveling at a medium to high speed can be stabilized to improve vehicle stability and the ease of driving.
The front/rear wheel steering method includes the mechanical transmission method achieved by mechanically linking the steering systems for the front wheels and the rear wheels and the method that enables adjustment of the front wheel steering and the rear wheel steering to be implemented independently of each other without mechanically linking the steering systems for the front and rear wheels to the steering wheel. In a vehicle with front/rear wheel steering angle control apparatus adopting the front/rear wheel steering method, the front and rear wheels may be steered to equal degrees in reverse phase relative to each other corresponding to the steering operation quantity in order to reduce the vehicle turning radius.
However, when parallel parking or turning a vehicle equipped with a front/rear wheel steering angle control apparatus in the related art while the vehicle is traveling forward, the vehicle manifests behavior in which part of the vehicle must be maneuvered toward the outside of the turn to a great degree. Following examples arises in countries in which the vehicle must keep to the left. FIGS. 15xcx9c17(a) and (b) present examples of the results of simulations of the behavior of the vehicle manifesting in the related art.
FIG. 15(a) presents an example of vehicular behavior manifesting when a vehicle in which the front wheels alone are steered (hereafter referred to as example 1) is parallel parked while it is traveling forward. In this situation, it is necessary to maneuver the vehicle to set the front left side of the vehicle outward to a great degree and thus, there arises a problem in that the vehicle cannot be set tightly at the side of the road (to the left) with ease during the parking operation.
FIGS. 16(a) and 17(a) present examples of vehicular behavior manifesting when the vehicle in example 1 is turned. As shown in the figures, when the vehicle is turned to the right, for instance, vehicular behavior manifests in which the front left side of the vehicle juts outward to a great degree. Thus, the driver must pay close attention to ensure that the front left side of the vehicle does not collide with an obstacle while turning the vehicle in a particularly narrow space.
FIG. 15(b) presents an example of vehicular behavior manifesting when a vehicle having the front wheels and the rear wheels steered to equal degrees in reverse phase in relation to each other (hereafter referred to as example 2) is parallel parked while it is advancing forward. During the initial stage of the parking procedure (when the vehicle is turned to the left in the figure), the rear right side of the vehicle juts out and thus, the driver must pay close attention to prevent the rear right side of the vehicle from colliding with an obstacle. At the final stage of the parking procedure (when the vehicle is turned to the right in the figure) during which the front left side of the vehicle juts out, a problem arises in that the vehicle cannot be maneuvered to the left with ease for parking.
FIGS. 16(b) and 17(b) present examples of vehicular behavior manifesting when the vehicle in example 2 is being turned. As shown in the figures, when the vehicle is turned to the right, for instance, a vehicular behavior manifests in which the front left side or the rear left side of the vehicle juts out to a great degree even during the initial stage of the turning operation. In particular, when turning the vehicle within a small space, the driver must pay close attention to ensure that the front left side and the rear left side of the vehicle do not collide with an obstacle.
An object of the present invention is to provide a vehicle front/rear wheel steering angle control apparatus and a vehicle front/rear wheel steering angle control method that allow a vehicle traveling forward to be parallel parked or to be turned in a smooth manner without any part of the vehicle allowed to jut out.
In the vehicle front/rear wheel steering angle control apparatus according to the present invention, which includes a steering angle control device that implements control on steering angles of front wheels and rear wheels independently of one another, a target steering angle calculating device that calculates target steering angles for the front and rear wheels and a steering angle adjustment device that adjusts the steering angles for the front and the rear of the vehicle so as to match the steering angles with the target steering angles calculated by the target steering angle calculating device, the target steering angle calculating device judges as to whether the vehicle is in a turn-contracting state, in which the direction along which a change made in the steering operation matches the direction of the steering operation or a turn-expanding state in which the direction along which the change is made in the steering operation is the opposite of the direction of the steering operation, calculates target steering angles for the front and rear wheels so as to allot a greater steering quantity for the front wheels compared to the rear wheels if it is decided that the vehicle is in a turn-contracting state and calculates target steering angles for the front and rear wheels so as to allot a greater steering quantity for the rear wheels compared to the front wheels if the vehicle is determined to be in a turn-expanding state.
The target steering angle calculating device may comprise a reference steering angle calculating device that calculates a reference steering angle achieved when the steering operation quantity is sustained at a constant value in correspondence the steering operation quantity, a steering operation intermediate quantity calculating device that calculates a steering operation intermediate quantity so as to gradually match the steering operation intermediate quantity with the quantity of the steering operation performed by the driver each time a specific fixed reference point on the vehicle shifts, a turn-contracting/turn-expanding judging device that judges as to whether the vehicle is in a turn-contracting state or a turn-expanding state based upon whether or not the direction of a steering operation corresponding to the steering operation intermediate quantity matches the direction of a change made in the steering operation and a reference steering angle correction device that obtains a target steering angle by correcting the reference steering angle so as to achieve a smaller steering center elevation angle compared to the steering center elevation angle achieved when the steering operation quantity is sustained at a constant value if it is decided by the turn-contracting/turn-expanding judging device that the direction of the steering operation corresponding to the steering operation intermediate quantity matches the direction of the change made therein and obtains a target steering angle by correcting the reference steering angle so as to achieve a larger steering center elevation angle if it is decided that the direction of the steering operation corresponding to the steering operation intermediate quantity is opposite from the direction of the change made therein.
In the vehicle front/rear wheel steering angle control method according to the present invention, which includes judging as to whether the vehicle is in a turn-contracting state in which the direction of a steering operation matches the direction of a change made in the steering operation or a turn-expanding state in which the direction of the steering operation is opposite from the direction of the change made therein, calculating target steering angles for front and rear wheels so as to allot a larger steering quantity to the front wheels compared to the rear wheels if the vehicle is determined to be in a turn-contracting state, calculating target steering angles for the front and rear wheels so as to allot a larger steering quantity to the rear wheels compared to the front wheels if the vehicle is determined to be in a turn-expanding state, and adjusting the steering angles at the front wheels and the rear wheels independently of one another so as to match the steering angles with the target steering angles that have been calculated.